1. Field of the Invention
This invention relates to an electrochemical device for measuring the concentration of an analyte in a biological fluid; more particularly, to a mechanism for determining the time at which the fluid provides an electrical connection between working and reference electrodes of the device.
2. Description of the Related Art
A variety of medical diagnostic procedures involve tests on biological fluids, such as blood, urine, or saliva, to determine an analyte concentration in the fluid. Among the analytes of greatest interest is glucose, and dry phase reagent strips incorporating enzyme-based compositions are used extensively in clinical laboratories, physicians, offices, hospitals, and homes to test samples of biological fluids for glucose concentration. In fact, reagent strips have become an everyday necessity for many of the nation's estimated 16 million people with diabetes. Since diabetes can cause dangerous anomalies in blood chemistry, it can contribute to vision loss, kidney failure, and other serious medical consequences. To minimize the risk of these consequences, most people with diabetes must test themselves periodically, then adjust their glucose concentration accordingly, for instance, through diet control and/or with insulin injections. Some patients must test their blood glucose concentration as often as four times or more daily.
It is especially important for people with diabetes who must control their diet in order to regulate sugar intake and/or administer insulin injections, and who must be guided in this regard by frequent tests of blood glucose concentration, to have a rapid, inexpensive, and accurate system for glucose determination.
One type of glucose measurement system operates electrochemically, detecting the oxidation of blood glucose on a dry reagent strip. The reagent generally includes an enzyme, such as glucose oxidase or glucose dehydrogenase, and a redox mediator, such as ferrocene or ferricyanide. This type of measurement system is described in U.S. Pat. No. 4,224,125, issued on Sep. 23, 1980, to Nakamura et al.; U.S. Pat. No. 4,545,382, issued on Oct. 8, 1985, to Higgins et al.; and U.S. Pat. No. 5,266,179, issued on Nov. 30, 1993, to Nankai et al., incorporated herein by reference.
Electrochemical glucose meters can be characterized as coulometric, amperometric, or potentiometric, depending on whether the system involves measuring charge, current, or potential, respectively, in making the determination of glucose concentration. In each case, it is important to define the point in time when the blood sample contacts the reagent, since an electrical signal must be applied to the strip at a precisely timed period thereafter.
Nankai et al., U.S. Pat. No. 5,266,179, issued on Nov. 30, 1993, discloses an electrochemical system for measuring blood glucose, in which the sample application time is defined as the time of a resistance drop between a pair of electrodes to which a constant voltage was applied.
White et al., U.S. Pat. No. 5,366,609, issued on Nov. 22, 1994, describes the same principle of monitoring the resistance drop between the electrodes to determine the time at which blood was applied to a dry glucose reagent strip. In both patents, a constant voltage is applied between working and reference electrodes to track resistance changes that result from the introduction of a blood sample to a dry reagent strip.
For accurate results, the sample detection procedure should not perturb the analyte concentration, and several techniques for minimizing analyte perturbation have been described.
Quade et al., German (DDR) Patent Application 148,387, filed on Dec. 28, 1979, discloses an electrochemical measurement that uses a novel electronic circuit, which allows rapid switching between potentiostatic (constant applied voltage) and galvanostatic (constant applied current) modes, while also allowing a reduction in the number of electronic components. A goal of the circuit is to minimize perturbation of the sample before the start of a measurement.
Bartels et al., German (DDR) Patent Application 208,230, filed on Nov. 24, 1981, discloses an electrochemical measurement that also attempts to minimize sample perturbation. The measurement device includes a circuit that uses a diode to minimize current flow before the start of the measurement, without using an additional amperometric control loop. Furthermore, the circuit switches to the potentiometric mode in a precise and rapid fashion.
Littlejohn et al., U.S. Pat. No. 4,940,945, issued on Jul. 10, 1990, discloses a portable apparatus that can measure the pH of a blood sample. The apparatus detects the presence of a sample in a cell by injecting a constant current between a fill electrode outside the sample chamber and one of two electrodes inside the chamber. When the impedance decreases by at least two orders of magnitude, the meter recognizes that sufficient sample has been provided and emits a beep. The fill electrode is then cut out of the circuit that includes the two electrodes inside the sample cell, and measurements are made potentiometrically.